Attenuator circuits are used in a variety of situations to control the amplitude of an analog signal provided to the input of a subsequent circuit. For example, analog to digital converters (ADCs) convert received analog signals within a defined signal range, and analog front end signal conditioning circuits with attenuators may be used to adapt an input signal to best use the conversion range of the ADC. Automatic gain control (AGC) circuits may be used in connection with adjustable attenuator circuits to dynamically set an attenuation value to optimize the usage of the ADC input range for varying input signal amplitudes. Step attenuators provide for attenuation adjustment in steps or increments, allowing digital implementation of programmable attenuators and AGC circuits. Digital step resistor attenuators are generally of two forms, including series and parallel architectures.
FIG. 7 shows a series resistor step attenuator circuit 700 that receives an input signal from an AC signal source 702 having source resistances RS. The attenuator 700 includes resistive divider circuits for attenuating a positive or plus signal IN+ and a negative or minus input signal IN−. The upper divider circuit includes a first resistor R1A and a resistor ladder circuit including series-connected resistors with individual values R. Similarly, the lower divider circuit includes a first resistor R1B connected in series with a number of resistors with individual values R. Switches individually connect midpoints between the series connected resistors of each ladder circuit with a corresponding output line OUT+, OUT−. The resistive divider circuit 700 allows programmable attenuation of the input signal VIN to provide an output signal at the lines OUT+ and OUT−. The series resistive step attenuator circuit 700 in FIG. 7 provides a generally constant input port voltage reflection coefficient scattering parameter, or “S-parameter” value S11. However, the series step attenuator 700 suffers from switch reliability problems, particularly for higher attenuation settings. In particular, the highest attenuation setting involves closure of the lowest switches in each of the ladder circuits. In this condition, the uppermost switch in each ladder circuit sees a relatively large voltage swing. For field effect transistor switches, this means that the upper switch transistor sees a much larger drain-source voltage swing VDS, gate-source voltage swing VGS and gate-drain voltage swing (VGD) than do the other switches. As a result, the upper switch may experience reliability problems, and due to large voltage swings across upper switches, there will be parasitic capacitive non-linearities that will degrade the performance of the attenuator.
FIG. 8 shows a parallel resistor step attenuator circuit 800 which receives an input signal from a signal source 802 with source resistances RS. The parallel attenuator 800 in FIG. 8 includes upper and lower first resistors R1A and R1B connected to the IN+ and IN− lines to receive the input voltage VIN. The circuit 800 also includes a number of parallel switched resistor branches connected between plus and minus output lines OUT+ and OUT−. In this example, each branch circuit includes a switch and a pair of resistors having a value R. Closing successively more of the switches of the parallel switch resistor branches causes a stepwise decrease in the amount of attenuation. The switches in the parallel attenuator circuit 800 typically do not suffer from reliability problems as was the case for the series attenuator 700. However, the reflection coefficient S11 varies significantly across attenuator settings in the parallel attenuator 800. In this regard, many applications have a maximum S11 specification, such as S11<−9 dB, and achieving this performance is difficult with the parallel attenuator 800 across all possible attenuation settings. In addition, resistance values become very small at high attenuation settings in the parallel configuration 800, and thus large switch sizes (e.g., transistor sizes) are necessary to reduce switch resistance. Larger switch size results in larger circuit area and larger parasitic capacitances of the larger transistors, leading to S11 degradation, and reduced input bandwidth. Because of this, operation at reduced S11 values is also difficult across all attenuation settings for the parallel step attenuator 800.